If you built a time machine and forced Alexander the Great's army to fight Genghis Khan's, who would win? The answer: It's not even close. A thousand years separate those two forces, and they include a few striking technological achievements.

Over at Slate's Quora blog, they break down the bloodshed, and it comes down to the invention of steel, better saddles, and gunpowder. Some salient quotes:

In Alexander the Great's time, steel was either unheard of or
incredibly rare. (It's disputed.) In Genghis Khan's time, it was
mass-produced by the ton to arm and armor entire armies...

Riding a horse without a solid saddle means all of your weight is on the
animals' back. With a solid saddle, the weight of the rider is
distributed over a larger area to the horse's flanks...

At this point in time, Chinese soldiers made use of flame throwers,
shrapnel bombs, and breech-loading guns. At the height of Genghis Khan's
reign, he had vanquished the northern Chinese Jin dynasty and absorbed
their heavy cavalry and gunpowder armed infantry into his forces.

In other words, it's not even close. You have one army with steel weapons, good armor and flamethrowers, and the other army fighting with flimsy weapons and basically wearing no protection. The whole thing is a fascinating read. [Slate]